The invention concerns a method for qualifying a diamond on the basis of a measured light transmission through the diamond, whereby the diamond is irradiated by a light source which emits light having a certain wavelength, and the transmission of this light through the diamond is measured and compared to a reference transmission through a reference diamond.
Ornamental diamonds are in demand for their unique qualities, such as brilliance, fire and scintillation. Natural diamond is so valuable that considerable research is being carried out in order to offer synthetic alternatives. Moreover, more and more treatments are used to increase the market value of low-quality diamonds.
Natural diamonds can be discerned from imitations, synthetic diamonds and treated diamonds on the basis of their physical properties. Laboratories always use several tests on specific characteristics to discern natural diamonds from others.
Diamonds are made up of a lattice of carbon atoms which may be substituted by nitrogen atoms in certain places. The majority of natural diamonds comprise nitrogen in a state of aggregation. They are classified as type Ia. In exceptional cases, nitrogen predominantly occurs as an isolated impurity, what is called type Ib. Rare are natural diamonds having nitrogen concentrations smaller than approximately 30 ppm, called type II. In the case of type Ia, the form of aggregation is usually mentioned as well, i.e. “A” for a substitution in groups of two nitrogen atoms, called A centres, and “B” for a substitution in groups of four nitrogen atoms with a vacancy, called B centres. Type Ia is further divided in IaA, IaB and IaAB as a function of the aggregation forms which can be detected by means of Fourier transformation infrared spectroscopy (FTIR).
The A and B centres hardly contribute to the colour of the diamond in the visible part of the spectrum. However, they provide for a specific absorption in the infrared part of the spectrum, on the basis of which the concentrations can be determined.
Isolated nitrogens, called C centres, strongly contribute to the absorption in the visible part of the spectrum and bring about the typical yellow colour of diamonds.
Natural diamonds usually have a very low content of C centres, i.e. less than 1 ppm, since the nitrogen was able to aggregate all the time these diamonds were situated in the crust of the earth under high pressure and at a high temperature. The temperature however, is limited, namely between 900° C. and 1350° C., so that aggregated nitrogen does not, or hardly split up to isolated nitrogen.
U.S. Pat. No. 6,650,489 describes a viewer for discerning natural, synthetic and treated gem stones. Differences in transmission between various gem stones are visually observed. An adjustable iris must prevent that the user can observe any light which did not go through the gem stone. As the absorption is not standardised to the intensity of the light source, the position of the iris, the size of the stone and other absorbing colour centres, this arrangement is not suitable to accurately discern natural from synthetic or treated diamonds. Moreover, the limit value for the discernment is left to the interpretation of the user and it can only be done in the visible part of the spectrum.
U.S. Pat. No. 6,650,489 also mentions the use of a “phosphorescope” for observing transmission differences in short wave UV. The “SSEF Type II spotter and illuminator” is such an appliance. The diamond to be examined is hereby sealed with clay towards the holder, which is placed above a short wave UV source. A phosphor screen makes it possible to visually observe the transmitted UV light. However, these appliances have numerous disadvantages such as: (i) it is difficult to guarantee the sealing, and it is impossible for the observer to notice this, (ii) the position of being fluorescent of the phosphor screen is to a large degree determined by the cut, such that for some cuts, such as for example the brilliant, only little light will leave the bottom side of the stone if the stone is being illuminated through the table facet, which is the most convenient arrangement in order to avoid leaking light, (iii) the fluorescence of some diamonds will illuminate the screen as well, which may lead to a wrong interpretation, (iv) additional measures must be taken in order to avoid that the observer is exposed to short wave UV, (v) low sensitivity, (vi) the limit value for the discernment is left to the interpretation of the user.
Further, U.S. Pat. No. 5,835,200 describes a method and device to positively identify natural diamonds by detecting what is called the zero phonon line at 415.5 nm, caused by what are called N3 centres, which are typically found in strongly aggregated nitrogen forms, such as in type IaAB diamonds. In order to be able to detect this highly selective absorption, the transmission at about 415.5 nm is scanned in the range of approximately 410 nm to 418.5 nm, by tilting a narrow-band filter. Several registrations including treatment are indicated to detect said narrow absorption peak with sufficient accuracy. The complexity of the arrangement and the wavelength range make this arrangement not suitable to be implemented in a cheap manner, in pocket size and/or fed by means of a battery.
Finally, patent EP 0 641 432 describes a method and appliance to positively identify natural diamonds by means of the registration of the transmission at two different central wavelengths, for example 254 nm and 365 nm. This method comprises the registration of the source intensity at said two wavelengths, and subsequently the registration of the transmission through the diamond at said wavelengths. A diamond is considered to be of the IaA or IaAB type, and consequently as being natural, if the transmission ratio related to the source intensity at 365 nm, namely from 20:1 to 100:1, is considerably larger than that at 254 nm, whereas for type IaB and type II, the ratio is about 2:1 to 1:1.
On the one hand, the transmission ratios related to the source intensity of 20:1 to 100:1 are linked to high concentrations of A centres, larger than 50 ppm, and thus clearly type IaA or IaAB diamonds. As the concentration of A centres is lower, said ratio will be reduced as well. On the other hand, the ratio 1:1 or 2:1 requires very low concentrations of A centres and no colour centres causing any additional absorption towards the deep UV, such as the yellow or brown colour, and thus the most clear crystals. It can be determined, by means of a UV-vis spectrometer, that this ratio may amount to 8:1 in a stone treated by General Electric, marketed under the name GE-POL. Consequently, below 20:1 it is impossible to give a decisive answer about a diamond being either or not natural or HPHT-treated. Measuring at two different wavelengths and the need of reference measurements require wavelength-selective parts and moving parts or a spectrophotometer, which increases the cost price. Consequently, this arrangement is not suitable to be implemented at low cost in pocket size.